See Zoom. This will allow you to adjust the text zoom level per website. If you want to change the default text zoom level for all websites you visit, have a look at the
NoSquint extension.

See [https://support.mozilla.com/en-US/kb/how-do-i-use-zoom?s=how+do+i+zoom+websites&as=s#w_how-do-i-use-text-zoom|Text Zoom]. This will allow you to adjust the text zoom level per website. If you want to change the default text zoom level for all websites you visit, have a look at the
[https://addons.mozilla.org/en-US/firefox/addon/nosquint/ NoSquint] extension.

'''CTRL + 0''' is to Reset Zoom
'''CTRL + +''' for Zoom In
'''CTRL + -''' for Zoom Out
You can also try the Default FullZoom Level 4.5 extension:
* https://addons.mozilla.org/en-US/firefox/addon/6965
Check and tell if its working.